“My father was a complete Nazi,” says Horst von Wächter. His father’s great friend Hans Frank was the governor general of Poland during the Second World War, while Horst’s father, Baron Otto Gustav von Wächter, was only slightly less high up in the Nazi hierarchy. In this compelling documentary, their two sons, Niklas and Horst, go on a road trip around the scenes of their fathers’ crimes in Ukraine and Poland along with a third figure, top human rights lawyer Philippe Sands. A specialist in trying genocide cases, his existence represents a slur on their fathers’ efficiency. His grandfather was a Jew living in Lviv during the war and was the only one of a family of 80 to survive the Nazis’ Grossaktion.

This is a film which is packed with remarkable, jaw-dropping moments. We see them flicking through their childhood holiday snaps – skiing, hiking in the mountains, shopping in the Warsaw ghetto. In the first 15 minutes Niklas Frank tells us that his mother was so desperate to avoid being divorced by Hans, she petitioned Hitler and the Fuhrer forbade it until after the war.

While Niklas reviles his father, Horst still clings to the idea that his father was an honorable man who just got caught up in something that was out of his control. So the structure of the film is the strident, morally certain Frank trying to hector the bumbling, befuddled Von Wächter into accepting his father’s guilt. He doesn’t actually say his father was just obeying orders – his case is that his signature wasn’t actually on orders that sent millions to their death and that signifies that he didn’t agree with the policy.